This invention relates to an electric appliance with a positive temperature coefficient (PTC) heating member, and a method of operating such an appliance, which may, for example, be an electric hair curler.
PTC thermistors, a type of PTC heating member, are made of polycrystalline ceramic on a base of barium titanate by doping a small amount of rare earth element, e.g. yttrium (Y), lanthanum (La), etc. PTC thermistors of various shapes and specifications may, for example, be obtained from Ohizumi Manufacturing Co., Ltd. of Japan.
FIG. 1 of the accompanying drawings is a graph showing a typical electrical resistance/temperature relationship of a PTC thermistor. The electrical resistance of the PTC thermistor is measured at the ambient temperature at a voltage sufficiently low to avoid self-heating. The temperature at which the electrical resistance of PTC thermistor begins to increase rapidly is called the xe2x80x9ccurie temperaturexe2x80x9d (Tc), which is defined as the temperature at which the resistance value is twice that of the minimum resistance value (Rmin). For the particular thermistor whose resistance/temperature relationship is shown in FIG. 1, the temperature coefficient ox, between any two temperatures (T1, T2) is given by equation (1) below:                     α        =                  2.303          ⁢                                    log              ⁢                                                R                  2                                                  R                  1                                                                                    T                2                            -                              T                1                                              ⁢          100          ⁢                      %            ⁢                          /              ∘                        ⁢                          C              .                                                          (        1        )            
As the electric voltage applied to a PTC thermistor increases, the temperature of the PTC thermistor will rise slowly by self-heating. When the temperature approaches and eventually exceeds the curie temperature (Tc), the electric current will begin to decrease, as shown in FIG. 2, which shows the relationship between the electric current passing through the PTC thermistor relative to the applied voltage, at various ambient temperatures. As can be seen in FIG. 2, such a relationship is influenced by the ambient temperature. When the electric voltage is gradually increased, the temperature of the PTC will gradually increase by self-generated heat. When the temperature reaches around the curie temperature (Tc), it shows a negative current characteristic, namely that as voltage continues to increase, the electric current decreases. This is shown in more detail in FIG. 3, which shows the relationship between the electric current passing through the PTC thermistor with time.
It can be seen in FIG. 3 that when an electric voltage is applied to the PTC thermistor, there will be a attenuation of the current. Initially, a very large electric current will flow through the PTC thermistor. As the time of application of this voltage increases, the electric current will decrease sharply until it reaches a low level, whereupon it will remain relatively constant. This low level is well below the normal working current of a heat generating resistor, and therefore there is, in the long run, an advantage of using PTC thermistor for generating heat, in preference to a resistor.
However, the characteristic shown most clearly in FIG. 3 has hindered the use of PTC thermistors as heating elements in electric appliances with heating members, in particular those appliances with batteries (rechargeable or otherwise) for operating the heating members. As discussed above, when an electric voltage is applied to the PTC thermistor, a large electric current will initially be drawn from the power source to start up the PTC thermistor. In cases where the power source are batteries, each time of starting the electric appliance will significantly shorten the normal useful life of the batteries, as batteries are not designed to provide such a large flow of electric current. This cannot be adequately compensated, even if the electric current decreases with the passage of time to a low level.
It is thus an object of the present invention to provide an electric appliance with a PTC heating member, and a method of operating such an electric appliance, in which the aforesaid shortcoming is mitigated, or at least to provide a useful alternative to the public.
According to a first aspect of the present invention, there is provided an electric appliance with a positive temperature coefficient (PTC) heating member and at least a first electric power source, wherein said PTC heating member is adapted to be powered by said first electric power source and at least a second electric power source, characterized in that said PTC heating member is adapted to be powered by said second electric power source when said electric appliance is started, and to be subsequently powered by said first electric power source.
According to a second aspect of the present invention, there is provided a method of operating an electric appliance with a positive temperature coefficient (PTC) heating member and at least a first electric power source, including steps (a) of powering said PTC heating member by at least a second electric power source; (b) powering said PTC heating member by said first electric power source, characterized in powering said PTC heating member by said second electric power source when said electric appliance is started, and powering said PTC heating member by said first electric power source subsequently.
Embodiments of the present invention will now be described, by way of examples only, and with reference to the accompanying drawings, in which: